Positive displacement fuel pumps, such as gear rotor type fuel pumps have been widely used to pump various liquids including hydrocarbon fuels such as gasoline. These pumps utilize mating inner and outer gears which, when driven to rotate, produce enlarging and ensmalling chambers which draw fuel into the pump and discharge fuel under pressure from the pump. Prior gear rotor type fuel pumps have cavitation noise problems when used to pump hydrocarbon fuels such as gasoline due to the tendency of such fuels to form vapor when exposed to decreased pressures, such as at the fuel pump inlet, and increased temperature which can occur within a vehicle's fuel tank and fuel system. The liquid fuel in a vehicle's fuel tank can become heated up to or near the temperature required for the liquid fuel to vaporize as the vehicle is operated or remains stationary in hot weather conditions. Heated fuel can also be returned to the fuel tank from a hot engine fuel rail or a fuel pressure regulator or other device disposed adjacent a hot fuel rail or engine. Due to the increased temperature of the fuel and the low pressure at the fuel pump inlet, under some conditions, there can be as much as 60% fuel vapor by volume within the fuel pumping chambers of the fuel pump.
As the amount of fuel vapor increases, the noise of the fuel pump in operation increases and, the efficiency of the fuel pump drops as a lower flow rate of liquid fuel is discharged from the pump. The noise is due in great part to cavitation, or the collapsing of the vapor pockets within the fuel pump as the relatively high pressure adjacent the outlet of the fuel pump rapidly and somewhat violently collapses the vapor within fuel pumping chambers which are at a lower pressure. Each time this occurs, an audible noise is produced. In use, due to the relatively high speed at which the gears are rotated, this occurs at such a high frequency that a loud humming noise is produced from the fuel pump. This loud noise in operation is very undesirable and is an even greater problem when the fuel pumps are mounted within a vehicle fuel tank which tends to amplify the noise of the fuel pump.
Additionally, prior fuel pump constructions, such as that disclosed in U.S. Pat. No. 5,035,588 have a flexible seal disposed against the downstream face of the gear rotors. These pumps are useful and relatively economical to manufacture and assemble for most automotive application. However, in high output pressure applications, such as marine engine applications wherein the fuel pump output pressure may be 90 psi or greater, the pressure differential across the flexible seal adjacent the pump inlet tends to force the seal firmly against the rotating gears which increases the wear on the seal and reduces the durability and service life of the fuel pump.